Vehicle and method for controlling engine start in a vehicle

ABSTRACT

A vehicle and a method for starting an engine in a vehicle are provided. The vehicle has a motor/generator, a disconnect clutch disposed between the engine and the motor/generator, and a transmission disposed between the motor/generator and the vehicle drive wheels. The transmission includes an input clutch which is selectively engagable for facilitating torque transfer between the motor/generator and the vehicle drive wheels. When an engine start is requested, the motor/generator is operated, and a start mode for the engine is determined based on a number of vehicle parameters. The transmission input clutch is partially disengaged to at least partially isolate the vehicle drive wheels from engine torque disturbances when the engine is started. The disconnect clutch is engaged, and the engine is fueled, thereby facilitating torque production by the engine.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a vehicle and a method for controllingengine start in a vehicle.

2. Background Art

Hybrid electric vehicles (HEV's) utilize a combination of an internalcombustion engine with an electric motor to provide the power needed topropel a vehicle. This arrangement provides improved fuel economy over avehicle that has only an internal combustion engine. One method ofimproving the fuel economy in an HEV is to shutdown the engine duringtimes that the engine operates inefficiently, and is not otherwiseneeded to propel the vehicle. In these situations, the electric motor isused to provide all of the power needed to propel the vehicle. When thedriver power demand increases such that the electric motor can no longerprovide enough power to meet the demand, or if the battery state ofcharge (SOC) drops below a certain level, the engine must start quicklyand smoothly in a manner that is nearly transparent to the driver.

One method of controlling an HEV powertrain is described in U.S. Pat.No. 6,176,808 issued to Brown et al. on Jan. 23, 2001, and incorporatedherein by reference. Brown et al. describes an HEV powertrain thatincludes a clutch disposed between an engine and a motor, which isoperable to disconnect the engine from the motor. The powertraindescribed in Brown et al. also includes a transmission located on anoutput side of the motor. The transmission includes a number of gearsand clutches which allow the transmission to be operated at differentspeed ratios. A control method described by Brown et al. includescontrolling the slip of reverse and forward clutches within thetransmission during engine idle and vehicle launch. Brown et al. notesthat during vehicle launch, the engine can be shutdown and the entirelaunch powered by the electric motor. In this situation, the forwardclutch in the transmission is fully locked, and is not allowed to slip.Alternatively, Brown et al. notes that the engine can be kept runningand the forward clutch in the transmission allowed to slip, therebyallowing the motor to assist the launch and provide optimum launchperformance.

Starting an engine in an HEV can occur when the vehicle is operatingunder any one of a number of different conditions. Control of the enginestart may be different for different sets of conditions under which thevehicle is operating. In addition, starting the engine in an HEV when itis moving under the power of the electric motor, may cause a noticeable,and therefore undesirable, torque disturbance in the vehicle driveline.Therefore, a need exists for a vehicle, and a method for controllingengine start in a vehicle, that reduces or eliminates driveline torquedisturbances when the engine is started.

SUMMARY OF THE INVENTION

One advantage of the present invention is that it provides a vehiclehaving an engine and an electric machine, wherein starting the enginecan be controlled to reduce or eliminate driveline torque disturbances.

Another advantage of the present invention is that it provides a methodfor controlling engine start in a vehicle that reduces or eliminatesdriveline torque disturbances even when the vehicle is being propelledby the electric machine.

The invention also provides a method for starting an engine in a vehiclehaving an electric machine, a first clutch disposed between the engineand the electric machine for selectively connecting the engine to theelectric machine, and a second clutch disposed between the electricmachine and vehicle drive wheels. The second clutch is selectivelyengagable for facilitating torque transfer between the electric machineand the vehicle drive wheels. The method includes requesting an enginestart, and operating the electric machine. The second clutch ispartially disengaged if the second clutch is fully engaged when theengine start is requested. The partial disengagement of the secondclutch causes the second clutch to slip, thereby at least partiallyisolating the vehicle drive wheels from engine torque disturbances. Themethod also includes engaging the first clutch, thereby connecting theengine to the electric machine, and fueling the engine, therebyfacilitating torque production by the engine.

The invention further provides a method for starting an engine in avehicle having a transmission, an accelerator, an electric machine, afirst clutch disposed between the engine and the electric machine forselectively connecting the engine to the electric machine, and a secondclutch disposed between the electric machine and vehicle drive wheels.The second clutch is selectively engagable for facilitating torquetransfer between the electric machine and the vehicle drive wheels. Themethod includes requesting an engine start, operating the electricmachine, and determining a start mode for the engine. Determining thestart mode for the engine is based at least in part on at least one of aposition of the accelerator and a current transmission gear. A firstengine start mode includes the transmission gear being first gear orhigher, and the accelerator position being at least partially open. Themethod further includes facilitating slip in the second clutch when itis determined that the engine is in the first start mode when the enginestart is requested. The facilitating of slip in the second clutch atleast partially isolates the vehicle drive wheels from engine torquedisturbances. The first clutch is engaged, thereby connecting the engineto the electric machine, and the engine is fueled, thereby facilitatingtorque production by the engine.

The invention also provides a vehicle including drive wheels, an engine,a transmission, an electric machine operable to propel the vehicle andto rotate the engine, and a first clutch disposed between the engine andthe electric machine for selectively connecting the engine to theelectric machine. A second clutch is disposed between the electricmachine and the vehicle drive wheels. The second clutch is selectivelyengagable for facilitating torque transfer between the electric machineand the vehicle drive wheels. The vehicle also includes a control systemincluding at least one controller. The control system is configured torequest an engine start, operate the electric machine, and facilitateslip in the second clutch if the transmission is in first gear or higherwhen the engine start is requested. The facilitating of slip in thesecond clutch at least partially isolates the vehicle drive wheels fromengine torque disturbances. The control system is further configured toengage the first clutch, thereby connecting the engine to the electricmachine, and to fuel the engine, thereby facilitating torque productionby the electric machine.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a vehicle in accordance with thepresent invention;

FIG. 2 is a schematic diagram showing some of the elements shown in FIG.1, including details of a transmission;

FIG. 3 is a flowchart illustrating a method in accordance with thepresent invention;

FIG. 4 is a graph illustrating how various parameters of the vehicleshown in FIG. 1 change over time during a rolling start;

FIG. 5 is a graph illustrating how various parameters of the vehicleshown in FIG. 1 change over time during a launch start;

FIG. 6 is a graph illustrating how various parameters of the vehicleshown in FIG. 1 change over time during a creep start; and

FIG. 7 is a graph illustrating how various parameters of the vehicleshown in FIG. 1 change over time during a key start.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

FIG. 1 shows a schematic diagram of a vehicle 10 in accordance with thepresent invention. The vehicle 10 is an HEV, and includes an engine 12,an electric machine, or motor/generator (M/G) 14, and a transmission 16disposed between the M/G 14 and vehicle drive wheels 18. The M/G 14 canoperate as a motor to provide torque to the vehicle wheels 18, and canalso operate as a generator, receiving torque from the engine 12 and/orvehicle wheels 18, thereby charging a battery 20. The M/G 14 can also beoperated to rotate the engine 12 when a first clutch, or disconnectclutch 22 is at least partially engaged.

The vehicle 10 also includes a control system, shown in the embodimentof FIG. 1 as three separate controllers: a vehicle system controller(VSC) 24, an engine control module (ECM) 26, and a transaxle controlmodule (TCM) 28. As shown in FIG. 1, the ECM 26 is directly connected tothe engine 12, while the TCM 28 is connected to the M/G 14 and thetransmission 16. The three controllers 24, 26, 28 are connected to eachother via a controller area network (CAN) 30. Although the controlsystem of the vehicle 10 is embodied in FIG. 1 in three separatecontrollers, such a control system could be embodied in more or lessthan three controllers, as desired.

FIG. 2 shows the transmission 16 in detail, along with some of the othervehicle components. The transmission 16 includes an input shaft 32 thatreceives torque from the M/G 14, the engine 12, or the M/G 14 and theengine 12. The transmission input shaft 32 is operatively connected to afirst portion 34 of a second clutch, or forward clutch (FC) 36. Thefirst portion 34 of the forward clutch 36 is also the first portion of adirect clutch (DC) 38. The forward clutch 36 and the direct clutch 38each have respective second portions 40, 42 which are operativelyconnected to a respective torque element within the transmission 16.

The second portion 40 of the forward clutch 36 is operatively connectedto a first sun gear (S1) 44 while the second portion 42 of the directclutch 38 is operatively connected to a first ring gear (R1) 46. Asshown in FIG. 2, the first planetary gear set, which includes the ringgear 46, the sun gear 44, and planetary carrier (P1) 48, is operativelyconnected to a second planetary gear set. The second planetary gear setincludes a second sun gear (S2) 50, which is connected to a reverseclutch (RC) 52. As shown in FIG. 2, the reverse clutch 52, whichincludes a friction brake 53, is also operatively connected to thetransmission input shaft 32.

The second planetary gear set also includes a planet carrier (P2) 54which is connected on one side to the ring gear 46, and on the otherside to a low-and-reverse brake (L/R) 56. A ring gear 58 defines asprocket for a chain drive, indicated generally at 60. The chain drive60 drives a sprocket 62, which in turn, drives a sun gear (S3) 64 of athird planetary gear set. A ring gear (R3) 66 is grounded to a housingof the transmission 16, while planetary carrier (P3) 68 is attached todifferential gearing 70. The differential gearing 70 is operable totransfer driving torque to each of two axle half shafts 72, 74. Adetailed cross sectional view of one embodiment of a transmission, suchas the transmission 16, is shown in U.S. Pat. No. 6,585,066 issued toKoneda et al. on Jul. 1, 2003, which is hereby incorporated herein byreference. Also shown in FIG. 2 is a pump 76 which is operable toprovide pressure to the transmission clutches.

FIG. 3 shows a high level flowchart 78 illustrating a method inaccordance with the present invention. As discussed above, the presentinvention provides a method for starting an engine, such as the engine12 in the vehicle 10. Throughout the description of the flowchart 78,the components of the vehicle 10, shown in FIGS. 1 and 2, will be usedfor reference. At step 80, an engine start is requested. This requestcan be initiated by the VSC 24, based on a number of input signals. Forexample, driver determined parameters, such as brake pedal position andaccelerator position may be used by the VSC 24 to calculate an overalldriver demand. In addition, the battery SOC, as well as information fromthe transaxle, provided by the TCM 28, can be used to determine when anengine start is needed. When an engine start is needed, the VSC 24 will“request” the engine start by coordinating operation of various vehiclecomponents, such that the engine 12 will be started with little or nodriveline torque disturbance apparent to a vehicle occupant.

At step 82, the M/G 14 is operated, and as explained below, will be usedto start the engine 12. The steps that will be performed to start theengine 12 are dependent on the state of the vehicle operation at thetime the engine start is requested. Therefore, at step 84, adetermination is made as to the engine start mode. In the embodimentshown in FIG. 3, there are four different engine start modes, each ofwhich is described below in detail. It is worth noting that more thanfour, or less than four, engine start modes are contemplated by thepresent invention; however, using the four engine start modesillustrated in FIG. 3 provides an efficient and effective method ofimplementing the present invention.

One step in determining which engine start mode will be used, is todetermine the current gear of the transmission 16. At decision block 86it is determined whether the transmission 16 is in first gear orhigher—this includes reverse. The transmission 16 is in a gear that islower than first gear only if it is in “zero gear”. In general, the term“zero gear” refers to the situation when the vehicle 10 is slowly movingforward in a creep mode, which is facilitated by the slipping of theforward clutch 36 and/or the low-and-reverse brake 56. It is worthnoting, however, that one or more of the clutches 36,38,52 and/or thelow-and-reverse brake 56 may be slipping a small amount, even when thetransmission is in first gear or higher.

For purposes of implementing the present invention, the TCM 28 willdetermine the current transmission gear. For example, if thetransmission 16 shifts into first gear, the TCM 28 will consider it toremain in first gear until it shifts into another gear. First gear forthe transmission 16 is characterized by the substantial engagement ofthe forward clutch 36 and the low-and-reverse brake 56. Even if theforward clutch 36 and/or the low-and-reverse brake 56 are allowed toslip a small amount while the transmission 16 is in first gear, it willnot be considered in zero gear. Only when the transmission 16 hasshifted out of first gear into zero gear—which may be characterized by amarked increase in the amount of clutch slip—will the TCM 28 considerthe transmission to be in zero gear.

If it is determined at decision block 86 that the transmission 16 is infirst gear or higher, the engine 12 is in a first, or rolling, startmode—see block 88. In the rolling start mode, the forward clutch 36 mayfully engaged, or as previously noted, it may already have a smallamount of slip. When it is determined that the engine 12 is in a rollingstart mode when the engine start is requested, slipping of the forwardclutch 36 is facilitated. Thus, if the forward clutch 36 is alreadyslipping a small amount, it can be further disengaged, if needed, toincrease the slip. Conversely, if the forward clutch is fully engagedwhen the engine start is requested, facilitating slipping involvespartially disengaging the forward clutch 36—see step 90. By partiallydisengaging the forward clutch 36, the vehicle driveline, including thevehicle wheels 18, is at least partially isolated from engine torquedisturbances, so that starting the engine 12 may be imperceptible to avehicle occupant.

If, at decision block 86, it is determined that the transmission 16 isnot in first gear or higher, it is next determined whether it is in zerogear—see decision block 92. If the transmission 16 is in zero gear, itis next determined whether the accelerator is at least partiallyopen—see decision block 94. If, at decision block 94, it is determinedthat the accelerator is at least partially open, the engine 12 is in alaunch start mode—see block 96. A launch start mode occurs when theaccelerator is open, and the vehicle is at a near rest condition.

The primary difference between the launch start mode and the rollingstart mode, is that in the launch start mode, the forward clutch 36 isalready slipping significantly when the engine start request isreceived. Therefore, in contrast to step 90, where the forward clutch 36was partially disengaged from its previously fully engaged position, atstep 98, the forward clutch 36 is kept partially disengaged. Asexplained more fully below, the amount of slip that is allowed to occurwhen the forward clutch 36 is partially disengaged, can be controlled bycontrolling the pressure of the clutch 36 and the speed of the M/G 14.

Returning to decision block 94, if it is determined that the acceleratoris not open, it is next determined whether the engine 12 is in a third,or creep, start mode—see block 100. A creep start mode occurs when avehicle is in a drive idle state. Drive idle occurs when the vehicle isat rest with the accelerator pedal off—i.e., the accelerator is closed.In a conventional vehicle, the torque converter provides a small amountof torque into the transmission. Whenever the brake pedal is released,this driveline torque will cause the vehicle to roll slowly under flatroad conditions. This low level of torque is known as creep.

In an HEV, the engine is normally shutdown during drive idle. When thebrake pedal is released, the engine can be requested to start. Such astart is known as “creep start”. In the creep start mode, as in thelaunch start mode, the forward clutch 36 is already partiallydisengaged, and is therefore already slipping. Thus, all that needs tobe done to keep the vehicle drive wheels 18 at least partially isolatedfrom engine torque disturbances, is to keep the forward clutch 36partially disengaged—see step 102.

Finally, if it is determined at decision block 92 that the transmission16 is not in zero gear—i.e., it is in park or neutral—then the engine 12is in a fourth, or key, start mode—see block 104. A key start occurswhen the vehicle operator turns the key in the vehicle to command theengine to start. This event replicates the start that is done innon-hybrid vehicles, where the starter motor is engaged to start theengine while the transmission is in either park or neutral. In the caseof the vehicle 10, the M/G 14 is used to rotate the engine 12 and bringit up to speed. Because the vehicle 10 is in park or neutral during thekey start mode, the forward clutch 36 is fully disengaged when the M/G14 is operated.

It is worth noting that even though the method illustrated in FIG. 3discusses keeping the forward clutch 36 less than fully engaged in orderto isolate the vehicle drive wheels 18 from engine torque disturbances,the same is true for the other input clutches in the transmission16—i.e., the direct clutch 38 and the reverse clutch 52. When an enginestart is requested, and it is determined that the engine 12 is in thekey start mode, the forward clutch 36 is kept fully disengaged—see block106. Having the forward clutch 36, and the other input clutches 38, 52,fully disengaged, completely isolates the vehicle drive wheels 18 fromany engine torque disturbances.

Regardless of which of the four engine start modes is used, thedisconnect clutch 22 is engaged so that the M/G 14 begins to rotate theengine 12 to bring it up to speed—see block 108. Finally, at block 110,the engine 12 is fueled, and it begins to produce torque. Although thesteps of the flowchart 78 described above in a particular order, inpractice, these steps need not be performed in any particular sequence,and in fact, one or more of the steps may be performed concurrently.

Each of the four engine start modes are now described in detail usingFIGS. 4-7. FIG. 4 shows a diagram of the rolling start mode as variousvehicle parameters change over time. Each of the diagrams shown in FIGS.4-7 are broken into six modes, modes 0-5. These modes provide aconvenient way to mark events throughout the various types of enginestarts. As shown in FIG. 4, the rolling start mode begins with the inputclutch pressure—i.e., the pressure in the forward clutch 36—relativelyhigh. This is because the forward clutch 36 is either fully engaged atthe beginning of the rolling start mode, or it is slipping a very smallamount. As shown in FIG. 4, the “Input Clutch Slip Speed” has some smallpositive value at the start of mode 0. This indicates that for therolling start illustrated in FIG. 4, the forward clutch 36 is alreadyslipping a small amount when the rolling start mode is entered.

During mode 0, the forward clutch 36 is partially disengaged to isolatethe driveline from engine torque disturbances. This is indicated by theincrease in input clutch slip speed. When the forward clutch 36 ispartially disengaged, and the M/G 14 is operating, there will be adifference in the angular speed of the first and second portions 34, 40of the forward clutch 36. This difference in speed is the slip speed ofthe forward clutch 36.

During mode 0, a target slip speed is set; this is shown by thehorizontal dot-dash line. If the actual slip speed is lower than thetarget slip speed, the pressure command to the forward clutch 36 isramped down. If the actual slip speed is higher than the target slipspeed, the pressure of the forward clutch 36 is held constant. There isalso a safety net used if the slip speed gets too high, in which case,the pressure command will be increased. By controlling the pressure ofthe forward clutch 36, and the speed of the M/G 14, the slip speed ofthe forward clutch 36 is increased until it is greater than a firstpredetermined slip speed. This is shown at the end of mode 0, at pointA, in FIG. 4.

In mode 1, the disconnect clutch 22 begins to be applied. The pressureof the disconnect clutch 22 is raised stepwise to a first level for apredetermined time—in this case, the duration of mode 1. This operationis known as boosting, and is used to fill the clutch 22 as fast aspossible. At the end of mode 1, the pressure in the disconnect clutch 22is reduced to a second level, and before the end of mode 2, it isincreased to an intermediate level between the first and second levels.This increase in the pressure of the disconnect clutch 22 during mode 2,helps to ensure that the M/G 14 provides enough torque through theclutch 22 to overcome the first compression stroke of the engine 12.

Also shown in FIG. 4, the M/G 14 is operated in torque control mode forthe duration of modes 0-2. During torque control mode, the M/G 14 isallowed to operate at whatever speed is necessary for it to produce adesired amount of torque. Also shown in FIG. 4, the torque of the M/G 14increases throughout modes 0-2. Near the end of mode 2, the speed of theengine 12 begins to increase. The end of mode 2, and the end of torquecontrol for the M/G 14, occurs when the engine speed is greater than afirst predetermined engine speed. This is shown at the end of mode 2, atpoint B, in FIG. 4.

In mode 3, the engine speed is brought up to the speed of the M/G 14,and the forward clutch 36 begins to lock. At the beginning of mode 3,the target slip speed—again indicated by the horizontal dot-dashline—for the forward clutch 36 is lowered, since some slip is stillneeded to provide driveline isolation, but inertia torque to assist theM/G 14 in turning the engine 12 is no longer needed.

Throughout mode 3, and part of mode 4, the M/G 14 is operated in speedcontrol mode. In speed control mode, the M/G 14 is controlled tomaintain a desired speed, and its output torque is allowed to fluctuate.As shown in FIG. 4, the torque of the M/G 14 generally decreasesthroughout modes 3 and 4, as the speed of the engine 12 increases. TheM/G 14 is operated in the speed control mode until its output torquefalls below a predetermined output torque. This is shown in FIG. 4 atpoint C. After the torque of the M/G 14 falls below the predeterminedoutput torque, the M/G 14 is again operated in torque control mode. Atthis point, the torque command is ramped to a negative value to dampenthe engine start transient and to begin charging the battery 20.

The end of mode 4 occurs when the slip speed of the forward clutch 36drops below a second predetermined slip speed. This is shown at point Din FIG. 4. After mode 4, control of the M/G 14 is transitioned back tothe VSC 24 and/or the TCM 28, and the engine start is finished. In mode5, the forward clutch 36 is fully engaged, thereby eliminating slip inthe forward clutch 36 and facilitating torque transfer from the engine12 and the M/G 14 to the vehicle drive wheels 18.

FIG. 5 shows the details of a launch start mode. As shown in FIG. 5,this start mode is similar to the rolling start mode illustrated in FIG.4, with the primary difference being that the forward clutch 36 isalready slipping significantly when mode 0 begins. Since the forwardclutch 36 is already slipping, the slip speed may already be above thetarget slip speed, which means the transition to mode 1 may be almostinstantaneous.

Similar to a launch start, the creep start mode begins with the forwardclutch 36 already slipping; this is illustrated in FIG. 6. At thebeginning of mode 0, the M/G 14 is not initially operating. During mode0, the M/G 14 is operated in unidirectional speed control mode toincrease its speed until a first predetermined speed is reached. This isshown at the end mode 0, at point A, in FIG. 6. In mode 1, thedisconnect clutch 22 is commanded to fill. As shown in FIG. 6, thepressure profile for the disconnect clutch 22 is essentially the samefor each of the different start modes, including the key start modedescribed below. Also similar to the other start modes, mode 2 for thecreep start mode ends when the speed of the engine 12 is greater than apredetermined engine speed. This is shown at point B in FIG. 6. Unlikethe launch start mode and the rolling start mode, however, the creepstart mode continues to operate the M/G 14 in the speed control mode,rather than transitioning between torque control and speed controlmodes.

When the slip speed of the disconnect clutch 22 reaches a predeterminedvalue, it indicates that the end of mode 3 is near. This is shown atpoint C in FIG. 6, where the engine speed essentially matches the speedof the M/G 14. In mode 4, the pressure of the clutch 22 is commanded toits maximum value through an open loop command that is calibrated tosmoothly complete the lock-up of the clutch 22. Once the maximumpressure command is obtained, control of the clutch 22 is complete, andthe mode changes to mode 5, which indicates the end of the start event.

FIG. 7 is a diagram illustrating the key start mode. As seen by acomparison between FIGS. 6 and 7, the key start mode very closelyresembles the creep start mode in its implementation. One obviousdifference is that none of the input clutches, including the forwardclutch 36, are engaged during a key start. This is evidenced by the lackof input clutch pressure shown in FIG. 7. In this mode, the forwardclutch 36 is fully disengaged. As in the creep start mode, the M/G 14 isoperated in speed control mode throughout the duration of the key startmode, and the disconnect clutch 22 begins to be engaged when the speedof the M/G 14 reaches a first predetermined speed.

While the best mode for carrying out the invention has been described indetail, those familiar with the art to which this invention relates willrecognize various alternative designs and embodiments for practicing theinvention as defined by the following claims.

1. A method for starting an engine in a vehicle having an electricmachine, a first clutch disposed between the engine and the electricmachine for selectively connecting the engine to the electric machine,and a second clutch disposed between the electric machine and vehicledrive wheels, the second clutch being selectively engagable forfacilitating torque transfer between the electric machine and thevehicle drive wheels, the method comprising: requesting an engine start;operating the electric machine; partially disengaging the second clutchif the second clutch is fully engaged when the engine start isrequested, the partial disengagement of the second clutch causing thesecond clutch to slip, thereby at least partially isolating the vehicledrive wheels from engine torque disturbances; engaging the first clutch,thereby connecting the engine to the electric machine; and fueling theengine, thereby facilitating torque production by the engine.
 2. Themethod of claim 1, further comprising fully engaging the second clutchafter the engine is fueled, thereby eliminating slip in the secondclutch and facilitating torque transfer from the engine and the electricmachine to the vehicle drive wheels.
 3. The method of claim 1, thevehicle including a transmission having the second clutch disposedtherein, the second clutch including a first portion operativelyconnected to an input shaft of the transmission and a second portionoperatively connected to a torque element in the transmission, whereinthe second clutch slips at a slip speed defined as a difference betweenthe angular speed of the first and second portions, and wherein engagingthe first clutch begins when the slip speed is greater than a firstpredetermined slip speed.
 4. The method of claim 1, wherein engaging thefirst clutch includes increasing a pressure of the first clutch to afirst level, holding the pressure of the first clutch generally constantat the first level for a predetermined time, lowering the pressure ofthe first clutch to a second level, and increasing the pressure of thefirst clutch to an intermediate level between the first and secondlevels, thereby engaging the first clutch with enough pressure toovercome a first compression stroke of the engine.
 5. The method ofclaim 1, wherein operating the electric machine includes: operating theelectric machine in torque control mode until a speed of the engine isgreater than a first predetermined engine speed, operating the electricmachine in speed control mode when the engine speed is greater than thefirst predetermined engine speed and output torque of the electricmachine is not less than a predetermined output torque, the operating ofthe electric machine in speed control mode effecting a reduction in theoutput torque of the electric machine when the engine begins to producetorque, and operating the electric machine in torque control mode whenthe output torque of the electric machine falls below the predeterminedoutput torque.
 6. The method of claim 1, the vehicle including atransmission operatively disposed between the electric machine and thevehicle drive wheels, the method further comprising: keeping the secondclutch fully disengaged if the second clutch is fully disengaged whenthe engine start is requested, and wherein operating the electricmachine includes increasing the speed of the electric machine, andengaging the first clutch begins when the speed of the electric machineis greater than a first predetermined electric machine speed.
 7. Themethod of claim 6, wherein the electric machine is operated in speedcontrol mode.
 8. A method for starting an engine in a vehicle having atransmission, an accelerator, an electric machine, a first clutchdisposed between the engine and the electric machine for selectivelyconnecting the engine to the electric machine, and a second clutchdisposed between the electric machine and vehicle drive wheels, thesecond clutch being selectively engagable for facilitating torquetransfer between the electric machine and the vehicle drive wheels, themethod comprising: requesting an engine start; operating the electricmachine; determining a start mode for the engine based at least in parton at least one of a position of the accelerator and a currenttransmission gear, a first engine start mode including the transmissiongear being first gear or higher, and the accelerator position being atleast partially open; and facilitating slip in the second clutch when itis determined that the engine is in the first start mode when the enginestart is requested, the facilitation of slip in the second clutch beingeffective to at least partially isolate the vehicle drive wheels fromengine torque disturbances; engaging the first clutch, therebyconnecting the engine to the electric machine; and fueling the engine,thereby facilitating torque production by the engine.
 9. The method ofclaim 8, further comprising fully engaging the second clutch after theengine is fueled, thereby eliminating slip in the second clutch andfacilitating torque transfer from the engine and the electric machine tothe vehicle drive wheels.
 10. The method of claim 9, wherein a secondengine start mode includes the transmission gear being below first gear,and the accelerator position being at least partially open, the methodfurther comprising keeping the second clutch partially engaged when itis determined that the engine is in the second engine start mode whenthe engine start is requested, the second clutch being kept partiallyengaged at least until after the engine is fueled.
 11. The method ofclaim 10, the second clutch including a first portion operativelyconnected to an input shaft of the transmission and a second portionoperatively connected to a torque element in the transmission, whereinthe second clutch slips at a slip speed defined as a difference betweenthe angular speed of the first and second portions, and wherein engagingthe first clutch begins when the slip speed is greater than a firstpredetermined slip speed.
 12. The method of claim 10, wherein engagingthe first clutch includes increasing a pressure of the first clutch to afirst level, holding the pressure of the first clutch generally constantat the first level for a predetermined time, lowering the pressure ofthe first clutch to a second level, and increasing the pressure of thefirst clutch to an intermediate level between the first and secondlevels, thereby engaging the first clutch with enough pressure toovercome a first compression stroke of the engine.
 13. The method ofclaim 10, wherein operating the electric machine includes, operating theelectric machine in torque control mode until a speed of the engine isgreater than a first predetermined engine speed, operating the electricmachine in speed control mode when the engine speed is greater than thefirst predetermined engine speed and output torque of the electricmachine is not less than a predetermined output torque, the operation ofthe electric machine in speed control mode effecting a reduction in theoutput torque of the electric machine when the engine begins to producetorque, and operating the electric machine in torque control mode whenthe output torque of the electric machine falls below the predeterminedoutput torque.
 14. The method of claim 8, wherein a third engine startmode includes the transmission gear being below first gear, and theaccelerator position being closed, the method further comprising keepingthe second clutch partially engaged when it is determined that theengine is in the third engine start mode when the engine start isrequested, the second clutch being kept partially engaged at least untilafter the engine is fueled.
 15. The method of claim 14, wherein a fourthengine start mode includes the transmission gear being a park or neutralgear, the method further comprising keeping the second clutch fullydisengaged when it is determined that the engine is in the fourth startmode when the engine start is requested.
 16. The method of claim 15,wherein operating the electric machine includes increasing a speed ofthe electric machine, and wherein engaging the first clutch begins whenthe speed of the electric machine is greater than a first predeterminedelectric machine speed.
 17. The method of claim 16, wherein the electricmachine is operated in speed control mode.
 18. A vehicle includingvehicle drive wheels, the vehicle comprising: an engine; a transmission;an electric machine operable to propel the vehicle and to rotate theengine; a first clutch disposed between the engine and the electricmachine for selectively connecting the engine to the electric machine; asecond clutch disposed between the electric machine and the vehicledrive wheels, the second clutch being selectively engagable forfacilitating torque transfer between the electric machine and thevehicle drive wheels; and a control system including at least onecontroller, the control system being configured to request an enginestart, operate the electric machine, and facilitate slip in the secondclutch if the transmission is in first gear or higher when the enginestart is requested, the facilitating of slip in the second clutch beingeffective to at least partially isolate the vehicle drive wheels fromengine torque disturbances, the control system being further configuredto engage the first clutch, thereby connecting the engine to theelectric machine, and to fuel the engine, thereby facilitating torqueproduction by the electric machine.
 19. The vehicle of claim 18, whereinthe control system is further configured to engage the second clutchafter the engine is fueled, thereby eliminating slip in the secondclutch and facilitating torque transfer from the engine and the electricmachine to the vehicle drive wheels.
 20. The vehicle of claim 19,wherein the transmission includes an input shaft and a torque element,and the second clutch includes a first portion operatively connected tothe transmission input shaft, and a second portion operatively connectedto the transmission torque element, and wherein the control system isfurther configured to determine a slip speed of the second clutch and toengage the first clutch when the slip speed is greater than a firstpredetermined slip speed, the slip speed being defined as a differencebetween the angular speeds of the first and second portions.